The present invention relates to spinal fixation systems generally, but more particularly, to an anterolateral spinal fixation system and a related method of insertion.
Anterior spinal approaches were described in the early 1900's by Hodgson, Ito and Royle. The use of anterolateral instrumentation, however, has always been, and remains controversial for fear of neurovascular and/or mechanical complications. On the other hand, anterior approaches have recognized advantages. For example, anterolateral instrumentation and fusion techniques address directly the site of instability or deformity and avoid the need for combined or staged posterior instrumentation and fusion. In addition, anterolateral instrumentation devices immobilize fewer motion segments than standard posterior hook fixation methods. Anterolateral instrumentation can also enhance stability in burst fractures, vertebral body tumors, post traumatic kyphosis, congenital kyphosis, and anterior opening wedge lumbar osteotomies. In any case, the optimal instrumentation needs to provide immediate postoperative stability, resist failure at bone/metal interfaces, and be relatively easy and safe to insert.
As indicated above, anterior and/or anterolateral devices have been used but have not been completely satisfactory. In some instances, the instrumentation lacks the necessary strength while, in others, proximity to the aorta leads to vascular problems in the form of aneurysms. Some devices do not permit independent screw placement and are usually statically locked in place after graft placement, occasionally leading to failure and oftentimes requiring further intervention.
The present invention seeks to overcome problems with prior anterolateral instrumentation by providing for independent screw placement combined with low profile instrumentation, thus avoiding the potential for device related neurologic and vascular injury; and by providing for relative movement, or dynamization, of the device after insertion, thus permitting continuous compression and load sharing and thereby avoiding microresorption.
In accordance with one exemplary embodiment of the invention, an anterolateral rod fixation device includes a pair of rods which can be fixed in substantially parallel relationship to the anterolateral vertebral body by cannulated fixation screws and associated locking screws. Upper or lower ends of the rods are provided with vertical slots to facilitate independent screw placement, and to permit relative movement between the rods and fixation screws upon removal of the locking screws. A pair of cross links are provided for rigid interconnection between the rods. Three types of cross links are contemplated for use with the fixation rods. In one version, slab-like cross links (which may be planar or slightly bent) are receivable within transverse slots provided in the rods, and rigidly secured in place by screws. In another version, sleeve-like "eyebolts" are fitted over the fixation rods, and the cross links are secured within transverse slots provided in the eyebolts. In the third version, shortened cross links are interconnected by a stepped coupling located between the fixation rods in order to accommodate the vertebral body contour without having to re-shape one or both of the fixation rods.
In accordance with another exemplary embodiment of the invention, a rod-plate fixation device includes one rod and a thin, lower profile plate in substantially parallel relationship, both of which may be fixed to the vertebral body substantially in the same manner as the double rod device of the first described embodiment. The plate offers a lower profile than the rod and, in addition, enhances stability by reason of a slightly wider stance. In this second embodiment, slab-like cross links may be secured directly to the plate, or by adaptations of the above described "eyebolts", as described in further detail hereinbelow.
In all cases, the fixation screw-bone interface may be enhanced by the utilization of four-pronged washers which are driven into the bone by the cannulated fixation screws. In addition, and as described below in greater detail, one or a pair of C-clips may be employed to limit the relative movement between and thus prevent complete distraction or separation of the fixation rods and the cannulated fixation screws.
Metals employed in the system are, of course, biocompatible, but where biodegradable components exist, they may be substituted for the metals, as will be understood by those skilled in the art.
Thus, in accordance with one aspect of the invention, there is provided an anterolateral spinal fixation system comprising a pair of substantially parallel fixation members, each provided with means for enabling varied fixation screw placement; a pair of cross links extending between the parallel fixation members, with the cross links extending through slots provided in the parallel fixation members and rigidly secured therein.
In accordance with another aspect of the invention, there is provided an anterolateral spinal fixation system comprising a pair of substantially parallel fixation members; a pair of cross links extending between the parallel fixation members; a pair of cannulated screws for each of the parallel fixation members adapted to secure the parallel fixation members to adjacent upper and lower vertebral body portions; and fastening means for rigidly securing first end portions of the fixation members to one of the upper and lower vertebral body portions, and for loosely securing second end portions of the fixation members to the other of the upper and lower vertebral body portions.
The present invention also relates to a process for insertion of the anterolateral device wherein the parallel fixation rods are initially secured at their lower ends to the lower vertebral body and, after the adjacent vertebrae on either side of an anterior portion to be replaced are separated to the extent necessary to permit insertion of the bone graft, the upper ends of the fixation rods are secured to the upper vertebral body, thus holding the adjacent vertebrae in the desired separated condition. After insertion and placement of the bone graft, the locking screws fix the upper ends of the fixation rods to the cannulated fixation screws are removed, thereby permitting relative movement between the upper fixation screws and the fixation rods which, in turn, permits continual bone graft compression during healing. As a result of this dynamization feature of the invention, solid fusion without implant failure has been achieved.
Other objects and advantages of the invention will become apparent from the detailed description which follows.